EL4585C
Horizontal Genlock, 8 F
SC
creases, T decreases. For LDET to be low at
i
Xtal Loop Filter Components (Approximate)
k
lock, T
50nS. C is used mainly to attenuate
4
l
i
l
Frequency
(MHz)
R2
R3
C3
C4
high frequency noise from the charge pump. The
effect these components have on time to lock is
illustrated below.
(kX)
(MX)
(pF)
(pF)
26.602
27.0
100
100
100
100
100
100
100
4.3
4.3
4.3
4.3
4.3
4.3
4.3
68
68
68
68
68
68
68
6.8
6.8
6.8
6.8
6.8
6.8
6.8
Lock Time
e
29.5
Let T R C . As T increases, damping increases,
3
3
35.468
21.476
24.546
28.636
but so does lock time. Decreasing T decreases
damping and speeds up loop response, but in-
creases overshoot and thus increases the number
of hunting oscillations before lock. Critical damp-
e
ing (g 1) occurs at minimum lock time. Because
decreased damping also decreases loop stability,
it is sometimes desirable to design slightly over-
l
PCB Layout Considerations
damped (g 1), trading lock time for increased
stability.
It is highly recommended that power and ground
planes be used in layout. The oscillator and filter
sections constitute a feedback loop and thus care
must be taken to avoid any feedback signal influ-
encing the oscillator except at the control input.
The entire oscillator/filter section should be sur-
rounded by copper ground to prevent unwanted
influences from nearby signals. Use separate
paths for analog and digital supplies, keeping the
analog (oscillator section) as short and free from
spurious signals as possible. Careful attention
must be paid to correct bypassing. Keep lead
lengths short and place bypass caps as close to
the supply pins as possible. If laying out a PCB
to use discrete components for the VCO section,
care must be taken to avoid parasitic capacitance
at the OSC pins 3 and 5, and FILTER out (pin
7). Remove ground and power plane copper
above and below these traces to avoid making a
capacitive connection to them. It is also recom-
mended to enclose the oscillator section within a
shielded cage to reduce external influences on the
VCO, as they tend to be very sensitive to ‘‘hand
waving’’ influences, the LC variety being more
sensitive than crystal controlled oscillators. In
general, the higher the operating frequency, the
more important these considerations are. Self
contained VCXO or VCO modules are already
mounted in a shielding cage and therefore do not
require as much consideration in layout. Many
crystal manufacturers publish informative litera-
ture regarding use and layout of oscillators which
should be helpful.
Typical Loop Filter
4585–16
LC Loop Filter Components (Approximate)
Frequency
(MHz)
R2
R3
C3
C4
(kX)
(kX)
(mF)
(mF)
26.602
27.0
100
100
100
100
100
100
100
39
39
43
51
30
36
43
0.01
0.01
0.01
0.01
0.01
0.01
0.01
0.001
0.001
0.001
0.001
0.001
0.001
0.001
29.5
35.468
21.476
24.546
28.636
11
ELANTEC [ ELANTEC SEMICONDUCTOR ]
